The preparation of platinum coatings on various substrates for use as catalysts have been the subject of many patents. A particularly important combination is platinum coated on tin oxide which is optically transparent.
In the prior art there is described in U.S. Pat. No. 4,273,624 a process for coating tin oxide with platinum by immersive tin oxide as an electrode in an electrolyte containing platinum ions and applying a potential which varies with time from positive to negative to positive. Another method of preparing a coating of platinum on tin oxide is described in an article entitled "Preparation of Dispersed Platinum On Conductive Tin Oxide And Its Catalytic Activity For Oxygen Reduction" by Watanabe et al. J. Electrochemical Society, Vol. 130, pages 59-64, January, 1983. In this method a tin oxide substrate is treated with a strong caustic solution to activate the substrate, immersed in a buffered solution of a platinum complex for a time sufficient for some of the platinum complex to bind to the substrate, and heating the substrate and bound platinum complex to a temperature of about 200.degree.-300.degree. C.
It has been found that the platinum coating prepared by the Watanabe et al. process is an agglomeration of platinum crystallites not approaching the ideal of a layer of platinum atoms. While the Watanabe et al. process is better than that previously known because it employs less platinum, there is much room for improvement to approach the ideal of an atomic layer.
An object of this invention is to provide an improved process for depositing a thin layer of platinum of a tin oxide substrate. It is another object of this invention to provide an improved process for depositing platinum on a tin oxide substrate by using a controlled potential chemisorption. Still other objects will appear from the more detailed description which follows.